Hi Bo,
On 06/07/18 17:27, Bo Yan wrote:
Hi Robin, Luke,
Recently I bumped into an error when running GCC undefined behavior sanitizer:
UBSAN: Undefined behaviour in kernel-4.9/arch/arm64/include/asm/checksum.h:34:6
ÂÂÂÂÂÂÂ load of misaligned address ffffffc198c8b254 for type 'const __int128 unsigned'
ÂÂÂÂÂÂÂ which requires 16 byte alignment
What's your config and reproducer here? I've had UBSan enabled a few times since that patch went in and never noticed anything. I've just tried it with 4.18-rc3 and indeed don't see anything from just booting the machine and making some network traffic. It does indeed fire if I also turn on CONFIG_UBSAN_ALIGNMENT, but then it's almost lost among a million other warnings for all manner of types - that's to be expected since, as the help text says, "Enabling this option on architectures that support unaligned accesses may produce a lot of false positives."
The relevant code:
ÂÂÂÂÂÂÂÂ tmp = *(const __uint128_t *)iph;
ÂÂÂÂÂÂÂÂ iph += 16;
ÂÂÂÂÂÂÂÂ ihl -= 4;
ÂÂÂÂÂÂÂÂ tmp += ((tmp >> 64) | (tmp << 64));
ÂÂÂÂÂÂÂÂ sum = tmp >> 64;
ÂÂÂÂÂÂÂÂ do {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ sum += *(const u32 *)iph;
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ iph += 4;
ÂÂÂÂÂÂÂÂ } while (--ihl);
But, I checked the generated disassembly, it doesn't look like anything special is generated taking advantage of that.
I'm using Linaro GCC 6.4-2017.08, expecting ldp instructions to be emitted, but don't see it.
My regular toolchain is currently Linaro 7.2.1-2017.11, but I also tried the last GCC 6 I had installed (6.3.1-2017.05), and for both at -O2 I see LDP emitted as expected for most of the identifiable int128 accesses (both in a standalone test harness and a quick survey of kernel code via 'aarch64-linux-gnu-objdump -S net/ipv4/*.o'). Of course, there may well be places where the compiler gets clever enough to elide all or part of that load where data is already held in registers - I've not audited *that* closely - but the whole point of having a pure C implementation is that it can be aggressively inlined more than inline asm ever could.
There were some prior discussions about GCC behavior, like this thread: https://patchwork.kernel.org/patch/9081911/ , in which you talked about the difference between GCC4 and GCC5.3. It looks to me this is regressed in Linaro GCC6.4 build.
I have not checked newer GCC versions.
Will it be more stable to just do this with inline assembly instead of relying on __uint128_t data type?
GCC documentation says __int128 is supported for targets which have an integer mode wide enough to hold 128 bits. aarch64 doesn't have such an integer mode.
Yet AArch64 GCC definitely does support __uint128_t, or this code wouldn't even build ;)
Robin.
Thanks
Bo